A response to a too good proposal. Asking if there's no catch and your not going to be expected to do something in return.
A shorter form of "no strings attached"
Dude 1: Here, you can borrow my car for your road trip.
Dude 2: For real! No strings?
This is a mythical act. No actual example of real "no Strings' has ever been witnessed.
No Strings is a concept that men think exist but actually doesn't. Certain women offer a "no strings" sex relationship to gullible males only to then proceed as it there are lots of strings. It is simply a honey trap, and an almost perfect one at that.
Millsy- "Elyssa offered me no strings fuck buddy sex so I went round and did her 3 times last night"
Lord Scotland - "I see, so how many times has she texted you today?"
Millsy - "456"